The present invention relates to outer space vehicles and more specifically to a cold vapor pressurization system that is used both during engine start and engine burn to provide the required engine net positive suction pressure (NPSP).
The current liquid propellant upper stage engine is pressure fed, meaning that the engine start thrust transient is directly proportional to tank pressure. The primary requirement for pressurization in support of engine start is to supply propellants to the engine at the required conditions (i.e., temperature and pressure). Net positive suction pressure is defined as the difference in total pressure and propellant vapor pressure at the engine inlet. A minimum NPSP value is established through engine performance tests. The tank pressurization system must guarantee engine NPSP start and steady-state levels.
The method presently used for providing engine NPSP is a high pressure (.apprxeq.3300 to 4000 psi) ambiently stored gaseous helium system. This helium is contained in multiple bottles which are manifolded together. These bottles are connected to the fuel and oxidizer tanks through tubing. Solenoid valves, coupled with fixed orifices, are cycled on and off to control the flow of helium from the bottles and maintain the appropriate tank pressure level to provide the required engine NPSP. Propellant tank pressurization is required for the pumped engine system to provide sufficient ullage pressure required to suppress cavitation in the engine turbopump.